Re: [SOLVED]Re: Only 8 MB/sec write throughput with NetBSD 5.1 AMD64

Hi All,

On 10/9/2011 2:12 PM, Thor Lancelot Simon wrote:
> On Sun, Oct 09, 2011 at 12:19:59PM -0700, Tony Bourke wrote:
>> changing MTU won't help really.
> Do you have data on which to base this conclusioN?

It's been long assumed that to get better throughput, enable jumbo 
frames (some switches support up to 12,000 byte frames, although jumbo 
frames is normally limited to 9,000 bytes). While that may have been 
true in the past (to tell the truth, I'd always parroted the assumption 
myself, never having tested it or seen any evidence), it doesn't seem to 
be true today.

Here are a couple of benchmarks to support that conclusion:


Jason Boche tests 9,000 byte frames with iSCSI, NFS and vMotion. The 
results were mixed, but none of the results showed Jumbo frames having a 
huge impact (best increase in performance was 7% for one metric, rest 
were nearly even).
http://www.boche.net/blog/index.php/2011/01/24/jumbo-frames-comparison-testing-with-ip-storage-and-vmotion/

NetApp and VMware also do joint performance reports comparing various 
protocols. They tested NFS and iSCSI with and without Jumbo frames 
(jumbo frames actually hurt iSCSI performance a bit). Overall, not a 
huge difference:

http://media.netapp.com/documents/tr-3808.pdf



>> it would also cause lots of other problems (like fragmentation over your
>> Internet connection)
> No.  That is what path MTU discovery is for.

MTU discovery doesn't work reliably. All you need is a site or admin 
along the path to disable ICMP.

http://en.wikipedia.org/wiki/Path_MTU_Discovery

Given that there's no real performance gain, and the problems that it 
can potentially cause with MTU mismatches, more and more networking 
administrators are choosing to leave it off these days.

>
>> and every device on your deity would need to be set for the same MTU.
> I am not sure what deity is involved with this, except perhaps for the god
> of unclear thinking (how many times have I tried to get him out of my
> personal pantheon? it's not working)

Sorry, I typed that on my phone while in a cab. Should have said "device".

>> Plus you should be able to push way more bandwidth without it.
> With a laptop disk drive as the data sink?
He'd been able to get higher throughput on Linux and Windows 7.

>
>> higher MTUs used to be a way to increase performance for things like
>> iSCSI but that's not really the case anymore, even with 10 Gigabit
>> Ethernet. So even 10 gigabit environments choose not to do jumbo frames.
>

>> If your network card is PCI, your bandwidth will be limited to about
>> 200-250 megabits per second (which is about 25-35 MBytes/s) because
>> of the bus speed.
> The claim is false.  Even giving a conservative 80MByte/sec estimate for
> 33MHz 32-bit PCI (the theoretical maximum is 132MByte/sec), and allowing
> for some adapter overhead, that's well north of 500 megabits per second
> -- and, in fact, 400-500 megabits per second can easily be achieved with
> such adapters, under NetBSD.  Clearly, either 64 bit or 66MHz PCI (never
> mind 64 bit, 133Mhz PCI-X) is sufficient to fill a gigabit link or come
> very, very close (and indeed it is easy to reproduce this result with
> real hardware under NetBSD as well).
You're right, my math was off. Still, PCI 32-bit can't max out a 1 
Gigabit link, especially considering it's a shared bus.

>> PCIe will handle full duplex 1 gigabit no problem.
> The claim is true.
>
> Thor

Tony